Chemical looping combustion or CLC: in the text hereafter, what is referred to as CLC (Chemical Looping Combustion) is an oxidation-reduction or redox looping method on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced.
In a context of increasing world energy demand, capture of carbon dioxide (CO2) for sequestration thereof has become an indispensable means to limit greenhouse gas emissions harmful to the environment. Chemical looping combustion (CLC) allows to produce energy from hydrocarbon-containing fuels while facilitating capture of the CO2 emitted upon combustion.
The CLC method consists in using redox reactions of an active mass, typically a metal oxide, for splitting the combustion reaction into two successive reactions. A first oxidation reaction of the active mass, with air or a gas acting as the oxidizer, allows the active mass to be oxidized. This reaction is usually highly exothermic and it generally develops more energy than the combustion of the feed. A second reduction reaction of the active mass thus oxidized, by means of a reducing gas from the hydrocarbon feed, then allows to obtain a reusable active mass and a gas mixture essentially comprising CO2 and water, or even syngas containing hydrogen (H2) and carbon monoxide (CO). This reaction is generally endothermic. This technique thus enables to isolate the CO2 or the syngas in a gas mixture practically free of oxygen and nitrogen.
The balance of the chemical looping combustion, i.e. of the two previous reactions, is globally exothermic and it corresponds to the heating value of the treated feed. It is possible to produce energy from this process, in form of vapour or electricity, by arranging exchange surfaces in the active mass circulation loop or on the gaseous effluents downstream from the combustion or oxidation reactions.
U.S. Pat. No. 5,447,024 describes for example a chemical looping combustion method comprising a first reactor for reduction of an active mass by means of a reducing gas and a second oxidation reactor allowing to restore the active mass in its oxidized state through an oxidation reaction with wet air. The circulating fluidized bed technology is used to enable continuous change of the active mass from the oxidized state to the reduced state thereof.
The active mass going alternately from the oxidized form to the reduced form thereof, and conversely, follows a redox cycle.
Thus, in the reduction reactor, active mass MxOy is first reduced to the state MxOy−2n−m/2 by means of a hydrocarbon CnHm that is correlatively oxidized to CO2 and H2O, according to reaction (1), or optionally to a mixture CO+H2, depending on the proportions used.CnHm+MxOy→nCO2+m/2H2O+MxOy−2n−m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state MxOy on contact with air according to reaction (2), prior to returning to the first reactor.MxOy−2n−m/2+(n+m/4)O2→MxOy  (2)
In the above equations, M represents a metal.
The efficiency of the circulating fluidized bed chemical looping combustion method is based to a large extent on the physico-chemical properties of the redox active mass.
The reactivity of the redox pair(s) involved and the associated oxygen transfer capacity are parameters that influence the dimensioning of the reactors and the rates of circulation of the particles. The life of the particles depends on the mechanical strength of the particles and on the chemical stability thereof.
In order to obtain particles usable for this method, the particles involved generally consist of a redox pair selected from among CuO/Cu, Cu2O/Cu, NiO/Ni, Fe2O3.Fe3O4, FeO/Fe, Fe3O4/FeO, MnO2/Mn2O3, Mn2O3/Mn3O4, Mn3O4/MnO, MnO/Mn, Co3O4/CoO, CoO/Co, or of a combination of some of these redox pairs, and sometimes a binder providing the required physico-chemical stability.
In general, and in particular in case of combustion of a gas feed of natural gas type, a high hydrocarbon conversion level, conventionally above 98%, is targeted while limiting the residence time of the reactants in the reaction zones in order to keep a realistic equipment size. These constraints require using a very reactive redox pair, which reduces the selection of the possible materials.
The NiO/Ni pair is often mentioned as the reference active mass for the CLC process for its oxygen transport capacities and its fast reduction kinetics, notably in the presence of methane. However, a major drawback is that nickel oxide has a high toxicity value, and it is classified as a CMR1 substance: Carcinogenic, Mutagenic or toxic for Reproduction of class 1, leading notably to significant constraints on the fumes filtration system. Furthermore, it has high cost. Indeed, since nickel oxide does not naturally occur with a sufficient concentration to allow interesting properties for the CLC process to be obtained, it is generally used concentrated in synthetic active mass particles whose manufacturing cost is high.
It can be noted that, in addition to the manufacturing cost, the cost of the active mass in a CLC process also involves the make-up active mass item, which can represent a significant part of the operating cost as a result of a continuous consumption of solid due to the inevitable attrition linked with the circulation of solid in the reactors where the gas velocities are relatively high. Thus, the cost related to the active mass is particularly high for synthetic particles, as opposed to particles obtained from some natural ores that can be less expensive.
Indeed, the use of natural ores as active mass for the CLC process, such as ilmenite or manganese ores, which can provide a satisfactory solution in terms of cost, is also known.
However, the use of such ores is generally less suited for combustion of gas feeds such as methane than for the combustion of solid or liquid feeds, in terms of process performance and of feed conversion ratio.
It is also known to use mixtures of natural metal oxides extracted from ores with nickel oxide.
Thus, a mixture of natural ilmenite (FeTiO3) and nickel oxide was experimented by Ryden et al. for methane combustion in a CLC process (“Ilmenite with addition of NiO as oxygen carrier for chemical looping combustion”, Rydén M. et al., Fuel 2010, 89, pp. 3523-3533). The behaviour of mixtures consisting of 95% ilmenite and 5% nickel oxide impregnated on various supports, among which an alumina support and a magnesium aluminate, was studied. These different solid mixtures were tested in the laboratory between 900° C. and 950° C. in two pilot plants, one operated in batch mode (single reactor) and the other in continuous combustion mode (active mass circulation loop between an air reactor and a fuel reactor). It appears that the presence of nickel oxide improves the feed conversion. However, significant transformations occur in the ilmenite structure, notably a decrease in density and an increase in the metal oxide porosity, likely to affect the lifetime of the particles. Particle agglomeration and sintering problems leading to CLC plant stoppage were also observed. These problems seriously call into question the advantage of using such a mixture as the oxygen carrier in a CLC process.
Using a mixture of natural hematite (Fe2O3) and nickel oxide was also tested within the context of a CLC process by Chen et al. (“Experimental investigation of hematite oxygen carrier decorated with NiO for chemical looping combustion of coal”, Chen D. et al., Journal of Fuel Chemistry and Technology 2012, 40, 267-272). The mixture circulates between an air reactor and a fuel reactor, both operated in fluidized bed mode. Besides, on the one hand, this study is limited to the combustion of coal and, on the other hand, it appears that the reactivity of the mixture differs depending on the mixture preparation methods, with the appearance of pore blockage in some cases. According to this study, the mixture of natural hematite and nickel oxide is obtained either by mechanical blending or by means of an impregnation method. In the case of impregnation of a solution of nickel nitrates on natural hematite, the mixture exhibits a low specific surface area suggesting that the effect on the reaction performances cannot be significant. Furthermore, impregnation on the natural hematite particles, showing an 80% Fe2O3 hematite content, allows to form nickel oxide particles on the hematite particles, but it also leads to a reaction with the elements already present so as to form a stable phase, such as NiAl2O4 spinel. Another unwanted effect of impregnation is the dissolution of a fraction of the iron by the impregnation solution (the Fe/Si ratio is modified). These two effects, the appearance of a stable phase and the dissolution of the oxygen-carrying phase, result in a decrease in the mass concentration of active sites providing oxygen to the system. In case of mechanical blending of the natural hematite and the nickel oxide, stability and sintering problems under reducing conditions, particularly linked with the use of natural hematite, are expected. In any case, the mixture studied by Chen et al. poses problems of chemical interaction with the refractory materials linked with the diffusion of iron at the temperatures used in CLC processes.
Another example is described in patent application WO-2014/068,205 relating to a CLC process using an active mass comprising a natural manganese ore of pyrolusite type enriched with nickel oxide, in order notably to improve the CLC process performances in terms of hydrocarbon feed conversion ratio. However, a major drawback of such a method is that the nickel oxide is part of the active mass that circulates in the chemical loop and produces fines through attrition, like the aforementioned CLC processes presented in the studies by Ryden et al. and Chen et al. The presence of nickel particles in the fines is unwanted due to the toxicity of nickel oxide, which limits the amount of nickel oxide that can be used and induces significant constraints on the filtration of the effluents resulting from the combustion.
There is therefore a need for an efficient CLC process, notably in terms of feed conversion, suited to the treatment of a gaseous hydrocarbon feed and that can use an inexpensive material for the redox active mass, complying with environmental standards in terms of toxicity and reducing emissions.